Horse behaviour is endlessly fascinating, whether we focus on the body language of the individual, or the herd. For those of you with the time for some extra reading, here is an interesting pamphlet from the University of Tennessee.
https://4-h.org/wp-content/uploads/2016/02/UnderstandingHorseBehavior.pd
https://4-h.org/wp-content/uploads/2016/02/UnderstandingHorseBehavior.pd